1. Field of the Invention
The present invention generally relates to labyrinth devices which provide velocity control of high pressure flowing fluids, both liquids and gasses, and more particularly to such devices which provide a total outlet area substantially larger than the total inlet area.
2. Description of the Prior Art
In the handling of flowing high pressure fluids, it has been customary to utilize orifice means having a high velocity short throat section to attain energy losses or high pressure drops. If the fluid is in a liquid state and liable to flash, that is, vaporize or turn to a gaseous condition on the downstream side of the orifice or valve opening, it may condense implosively and induce damaging shock waves, cause erosion, and the like. Also, as the velocity of the fluid in the valve exceeds the velocity of the fluid in the line, several disturbing reactions occur. The most serious problem is rapid erosion of valve seat plug by direct impingement of the liquid and any foreign particles suspended therein. Additional erosion results from cavitation. Cavitation may be defined as the high speed implosion of vapor against the valve trim and body.
In addition to the severe problems resulting from erosion, the increased velocity also causes the flow characteristics of the valve to become unpredictable and erratic. This occurs because the changes in velocity significantly affect the valve vena contracta vortexes and fluid enthalpies.
Other objectionable problems created by the high fluid velocity in the valve are severe noise generation, trim fatigue and possible degradation of flowing fluid materials such, for example, as polymers.
The foregoing and other deficiencies were somewhat overcome by more recent devices which affect dissipation of energy of a flowing high pressure fluid by subdividing it into a plurality of passageways and labyrinths in which there are rapid changes of direction. An example of such devices may be found in U.S. Pat. No. 3,514,074 and 3,513,864. These devices are formed as a series of cylindrically stacked members having inlets and outlets formed along concentric circular peripheries of each member with a labyrinth being formed therebetween. Where increased labyrinth outlet to inlet area ratios are required to provide a predetermined pressure drop through this device the circular outlet is appropriately increased by increasing the radius of each member to thereby increase the outlet area of the device. For high pressure drops the device becomes quite large and requires an increased amount of material and cost. Also, where a high temperature high pressure fluid is passed through such a device a significant temperature drop is encountered between the inlet and outlet due to the pressure drop through the device. This temperature differential may produce circumferential thermal stresses due to compressive forces on the high temperature inlet area and tensile stresses on the low temperature outlet area which may cause the members of the device to split and warp. When the members split or warp the labyrinth they form may be short-circuited thereby impairing the effectiveness and operation of the device. The foregoing problems become especially acute in atmospheric venting situations.
Atmospheric venting is generally the noisiest of all control valve applications. The problem is difficult to treat because of the inherently higher pressure ratios, mass flow rates and the absence of the pipe wall as an attenuator. Fluid borne noise downstream of control valves is very high. If not treated or contained with the pipe, this noise can result in sound pressure levels of 150 to 170 dB three feet from the vent exit. Sound sources of this magnitude are hazardous to personnel and frequently result in complaints from local residents.
Mufflers and silencers can only attenuate fluid borne noise 20 to 30 dB. Therefore, only partial success has been achieved with them in obtaining desired sound pressure levels. Furthermore, a typical path treatment system i.e., the muffler, lagging, support structure, etc., is very cumbersome and expensive. Often, the total cost of path treatment can exceed the valve cost many times over.